A mammogram showing a breast tumour. Around half of all women diagnosed with breast cancer each year could potentially benefit from the latest findings.
Photograph: Rui Vieira/PA

Hundreds of thousands of women with breast cancer could potentially benefit from having a low-cost female hormone added to their therapy, scientists say.

The fresh hope emerged from research in animals which found that the hormone, progesterone, slowed the growth of breast cancers when it was combined with tamoxifen, the standard drug treament.

Nearly 1.7 million women globally are diagnosed with breast cancer each year, and about half could stand to benefit from progesterone therapy if the findings are confirmed in clinical trials.

In the UK alone, about 25,000 women a year have breast tumours that could respond to the new therapy, scientists said.

Breast cancer could be 'stopped in its tracks' by new technique, say scientists

Read more

“The results are pretty clear and potentially have direct benefits for many women with breast cancer,” said Jason Carroll, who co-led the study at the Cancer Research UK Cambridge Research Institute.

Doctors have known for some time that levels of certain molecules in breast tumours have an impact on how well the cancer will respond to treatment, and the patient’s chances of survival. Women whose tumours have high levels of oestrogen receptor, for example, benefit from drugs that block the effects of the oestrogen hormone. The drugs work because they put a brake on the cell division and tumour growth driven by oestrogen.

But another molecule, the progesterone receptor, is also important. Women whose breast cancers are “double positive”, or have high levels of both oestrogen and progesterone receptors, have the best survival rates. Until now, the reason why was unknown.

Working with another team led by Wayne Tilley at the University of Adelaide, Carroll’s group performed a series of experiments on human breast cancer cells and on mice implanted with human breast tumours. They found that adding progesterone to double positive cancers, along with tamoxifen, slowed their growth more than the drug alone.

It turns out that when progesterone sticks to the progesterone receptor in cancer cells, it alters how the oestrogen receptor works, and effectively puts a second brake on tumour growth. Tests showed that mice given progesterone and tamoxifen had breast tumours only half the size of those given the drug on its own.

“Crucially, it provides a strong case for a clinical trial to investigate the potential benefit of adding progesterone to drugs that target the oestrogen receptor, which could improve treatment for the majority of hormone-driven breast cancers,” Carroll added. Details of the study are reported in Nature.

Tamoxifen and other drugs that target oestrogen are usually given to breast cancer patients after they have had surgery to remove their tumours. Emma Smith at Cancer Research UK, which funded the study, said that if progesterone is found to work in human trials, the hormone could particularly help women whose tumours have become resistant to standard drugs.

“This exciting study in cells shows how a cheap, safe, and widely available drug could potentially improve treatment for around half of all breast cancer patients,” she said. “Thanks to research, almost 70% of women now survive breast cancer beyond 20 years, but so much more must be done and we won’t stop until we find cures for all forms of the disease.”